1. Field of the Invention
The present invention relates in general to audio optical recording disks for recording audio signals as digital data, and to an encoder apparatus and decoder apparatus for recording and playback of data to/from such a disk. In particular, the invention relates to a new type of audio optical recording disk which is in accordance with DVD (digital video disk) standards and which enables selection of playback of recorded audio data as either PCM (pulse code modulation) data or as single bit stream data which has been generated by sigma-delta modulation, and to types of encoder apparatus and decoder apparatus which are suitable for use with such a disk.
2. Prior Art
More than ten years have passed since the CD (compact disk) was first marketed as an optical recording disk having audio signals recorded thereon in digital form, and CDs have already surpassed cassette tapes as the main medium used for audio recording. Various types of data (Eight to Fourteen) modulation and recording method which utilizes 8-bit fixed-length data symbols, and the CRC method, etc. CD players have been correspondingly developed, having various types of added application functions.
An audio signal is recorded on a CD by encoding as PCM data, using a sampling frequency of 44.1 kHz and 16 bits per sample (i.e. 16 quantization bits), for each of the left (L) and right (R) channel stereophonic signals. Such recording provides a reproduction bandwidth of approximately 22 kHz, and a S/N (signal-to-noise) ratio of approximately 96 dB. In the case of CD-ROMs which are used for example in the field of electronic publishing, audio data are generally recorded on such a disk by using ADPCM (Adaptive Differential Pulse Code Modulation) to compress the data. As a result, the reproduced sound quality obtainable is inferior to that obtainable by audio CDs.
In recent years, it has come to be realized that the playback characteristics of the conventional type of CD are unsatisfactory, with respect to the reproduction bandwidth (i.e. the bandwidth of the analog audio signal obtained by playback of data from such a disk) and the signal/noise ratio, and that there is a need to establish standards for a next-generation audio recording disk which will have superior characteristics. Basically, the requirements for the characteristics of such a new type of audio recording disk would be as follows: a reproduction bandwidth of 100 kHz and a signal/noise ratio that is in the range 120 to 144 dB.
One method which is receiving attention for the purposes of implementing such a type of audio disk is the single bit stream method, whereby a stream of successive single bits (as respective sample values) is produced as a result of A/D (analog-to-digital) conversion of an analog audio signal, rather than a stream of successive multi-bit sample values. This type of A/D conversion can be performed by sigma-delta modulation, for example as described in Japanese Patent Application Laid-open (HEI) 6-232755. FIGS. 24A to 24D are respective block system diagrams for illustrating different methods of A/D and subsequent D/A (digital-to-analog) conversion applied to analog audio signals, for transfer through a processing system such as an optical disk recording/playback system. FIG. 24D illustrates the sigma-delta modulation method for generating single bit stream data. As shown, conversion of an analog audio signal to a stream of single bit stream data is performed by first transferring the analog audio signal through a first or second-order analog LPF (low-pass filter) 200, then supplying the resultant signal to an analog sigma-delta modulator 201. In this example, the sigma-delta modulator 201 consists of an input subtractor, an integrator receiving the output signal from the subtractor, a 1-bit quantizer which operates at a fixed quantization frequency and operates on the output signal from the integrator, and a feedback delay element (Z.sup.-1) providing a delay amount which is equal to one quantization period, so that the subtractor subtracts the delayed output signal of the quantizer from the input signal of the integrator. Since the principles of sigma-delta modulation are now well known, detailed description will be omitted. The resultant output flow of successive 1 or 0-state bits (referred to herein as "single bit stream data) can be supplied directly to a recording system to be recorded on an optical recording disk, and playback of the optical recording disk can subsequently be performed. Such recording and playback operations are represented as a "processing system" 202. In this example, the sigma-delta modulation sampling frequency (i.e. the bit rate of the resultant data stream) is assumed to be 768 kHz.
By comparison with the conventional PCM method used for audio CDs which is shown in FIG. 24A, the oversampling method which is shown in FIG. 24B, or the analog sigma-delta modulation method which is shown in FIG. 24C (in which each sample that is obtained by the sigma-delta modulation process is encoded as a plurality of bits rather than as a single bit), the single bit stream type of sigma-delta modulation has the following advantages:
(1) The sampling frequency (i.e. the bit rate) can be made sufficiently high to enable very effective noise shaping to be achieved. As a result, an excellent signal/noise ratio can be attained. PA1 (2) By comparison with the conventional PCM method, the reproduction bandwidth can be made substantially wider. PA1 (3) With the single bit stream method, the part of a playback system which recovers an original analog audio signal from a stream of digital data can be made extremely simple, i.e. only an analog LPF is necessary. With the conventional methods of FIGS. 24A to 24C, on the other hand, it is necessary to utilize a D/A converter to implement that function. PA1 first and second sigma-delta modulators for respectively converting the L channel analog audio signal and R channel analog audio signal to first and second streams of single bit sequence data, PA1 data converter means for converting the first and second streams of single bit sequence data to respective first and second PCM data streams at the sampling frequency of 48 kHz and with the fixed number of bits per data sample, and PA1 formatting means for generating each of the audio data portions of respective audio packets by assigning respective bits of the first and second PCM data streams to the first and second data channels, and assigning respective bits of the first and second streams of single bit sequence data to the remaining 8 sets of the 10 data channels, for generating the ADI portion of the audio packet, containing the discrimination data, and for attaching the ADI portion to the audio data portion. PA1 first and second sigma-delta modulators for respectively converting the L channel and R channel analog audio signals to first and second streams of single bit sequence data, PA1 data converter means for converting the first and second streams of single bit stream data to respective first and second PCM data streams at the sampling frequency of 44.1 kHz and with the fixed number of bits per data sample, and PA1 formatting means for generating each of the audio data portions of respective audio packets by assigning respective bits of the first and second PCM data streams and the padding bits to the first and second data channels, and assigning respective bits of the first single bit sequence data and second single bit sequence data to the remaining 8 sets of the 10 data channels, for generating the ADI portion of the audio packet, containing the discrimination data, and for attaching the ADI portion to the audio data portion. PA1 mode setting means for selectively setting a PCM decoding mode and a single bit sequence decoding mode, PA1 discrimination means for detecting the discrimination data of the ADI portions of the playback data stream, and for judging the discrimination data to determine whether the optical recording disk has audio data recorded thereon in the supplementary stream mode, PA1 deformatting means functioning, when the discrimination means has determined that the optical recording disk has audio data recorded thereon in the supplementary stream mode, to separate the PCM data conveyed by the first and second data channels and the single bit sequence data conveyed by the remaining 8 data channels of the playback data stream into a first output data stream (SA) and second output data stream (SB) respectively, and for producing only the first output data stream (SA) when the PCM decoding mode has been set and producing only the second output data stream (SB) when the single bit sequence decoding mode has been set, PA1 first channel separator means (42) for separating and processing the first output data stream (SA) into a stream of PCM data corresponding to the L channel analog audio signal and a stream of PCM data corresponding to the R channel analog audio signal. PA1 first and second D/A (Digital-to-Analog) converter means (44, 45) for respectively converting the streams of PCM samples corresponding to the the L channel analog audio signal and R channel analog audio signal to the L and R channel analog audio signals respectively, PA1 second channel separator means (43) for separating and processing the second output data stream (SB) into a stream of single bit sequence data corresponding to the L channel analog audio signal and a stream of single bit sequence data corresponding to the R channel analog audio signal, and PA1 first and second low-pass filters (46, 47) for respectively converting the streams of single bit sequence data corresponding to the L channel analog audio signal and R channel analog audio signal to the L channel and R channel analog audio signals. PA1 wherein first and second ones of the 10 data channels respectively convey, encoded as respective PCM data streams with the fixed number of bits per data sample, first (Lt) and second (Rt) combination channel signals which in combination convey three front channel (L, C, R) analog audio signals and one rear channel (R) analog audio signal, with the front channel and rear channel analog audio signals having been encoded to form the first (Lt) and second (Rt) combination channel signals by the Dolby Surround encoding method, PA1 wherein a remaining 8 of the data channels convey the three front channel and single rear channel analog audio signals encoded by sigma-delta modulation as respective streams of single bit sequence data, and PA1 wherein each of the audio packets includes an ADI portion containing discrimination data specifying that the audio data portion of the audio packet has been recorded in the supplementary stream mode and specifying that the first and second combination channel signals have been generated by the Dolby Surround encoding method. PA1 sigma-delta modulators for converting the three front channel (L, C, R) analog audio signals and one rear channel (R) analog audio signal to respective ones of 4 streams of single bit sequence data, PA1 encoding means for encoding the streams of single bit sequence data to form the first and second combination channel signals (Lt, Rt) as respective streams of single bit sequence data, by the Dolby Surround encoding method, PA1 first and second data converter means for converting the first and second combination channel signals (Lt, Rt) to respective first and second PCM data streams at the sampling frequency of 48 kHz and with the fixed number of bits per data sample, PA1 bandwidth compression means for applying bandwidth compression by a factor of 1/2 to each of the 4 streams of single bit sequence data, and PA1 formatting means for generating each of the audio data portions of respective audio packets by assigning respective bits of the first and second PCM data streams to the first and second data channels and assigning respective bits of the 4 streams of single bit sequence data to the remaining 8 data channels, for generating the ADI portion of the audio packet, containing the discrimination data, and for attaching the ADI portion to the audio data portion. PA1 mode setting means for selectively setting a PCM decoding mode and a single bit sequence decoding mode, PA1 discrimination means for detecting the discrimination data of the ADI portions of the playback data stream, for thereby judging whether the optical recording disk has audio data recorded thereon in the supplementary stream mode and whether the audio data include the Dolby Surround encoded data, PA1 deformatting means functioning, when the discrimination means has determined that the optical recording disk has audio data recorded thereon in the supplementary stream mode and that the recorded data include the Dolby Surround encoded data, to separate the PCM data conveyed by the first and second data channels and the single bit sequence data conveyed by the remaining 8 data channels of the playback data stream into a first output data stream (SA) and second data stream (SB) respectively, and for producing only the first output data stream (SA) when the PCM decoding mode has been set and producing only the second output data stream (SB) when the single bit sequence decoding mode has been set, PA1 first channel separator means for separating and processing the first output data stream (SA) to obtain the first combination channel signal (Lt) and second combination channel signal (Rt) as respective PCM data streams, PA1 decoding means for decoding the first and second combination channel signals to obtain four PCM data streams respectively corresponding to the four channel (L, C, R, S) analog audio signals, PA1 digital-to-analog converter means for converting the four PCM data streams to the four channel (L, C, R, S) analog audio channel signals respectively, PA1 second channel separator means for separating and processing the second output data stream into four streams of single bit sequence data respectively corresponding to the four channel (L, C, R, S) analog audio signals, and PA1 low-pass filter means for converting the four streams of single bit sequence data to the four-channel (L, C, R, S) analog audio signals. PA1 wherein first and second ones of the 10 data channels convey, encoded as PCM data with the fixed number of bits per sample, a stream of combination audio data derived by encoding N-channel analog audio signals using a predetermined audio channel encoding method, where N is a fixed plural integer, PA1 wherein a remaining 8 of the data channels convey the combination audio data, encoded by sigma-delta modulation as respective streams of single bit sequence data, and PA1 wherein each of the audio packets includes an ADI portion containing discrimination data specifying that the audio data portion of the audio packet has been recorded in the supplementary stream mode and specifying that the packet conveys combination audio data which have been generated by the predetermined audio channel encoding method. PA1 sigma-delta modulators for converting the N-channel analog audio signals to respective streams of single bit sequence data, PA1 encoding means for encoding the streams of single bit sequence data to form the combination audio data, by the predetermined audio channel encoding method, PA1 data converter means for converting the combination audio data stream to a PCM data stream at the sampling frequency of 48 kHz and with the fixed number of bits per data sample, PA1 bandwidth compression means for applying bandwidth compression by a factor of 2/N to each of the streams of single bit sequence data, and PA1 formatting means for generating each of the audio data portions of respective audio packets by assigning respective bits of the PCM data stream to the first and second data channels and assigning respective bits of the each of the streams of single bit sequence data to the remaining 8 data channels, for generating the ADI portion of the audio packet, containing the discrimination data, and for attaching the ADI portion to the audio data portion. PA1 mode setting means for selectively setting a PCM decoding mode and a single bit sequence decoding mode, PA1 discrimination means for detecting the discrimination data of the ADI portions of the playback data stream, for thereby judging whether the optical recording disk has audio data recorded thereon in the supplementary stream mode and whether the audio data include combination audio data encoded by the predetermined audio channel encoding method, PA1 deformatting means functioning, when the discrimination means has determined that the optical recording disk has audio data recorded thereon in the supplementary stream mode and that the recorded data include the combination audio data encoded by the predetermined audio channel encoding method, to separate the PCM data conveyed by the first and second data channels and the single bit sequence data conveyed by the remaining 8 data channels of the playback data stream into a first output data stream and second data stream respectively, and for producing only the first output data stream when the PCM decoding mode has been set and producing only the second output data stream when the single bit sequence decoding mode has been set, PA1 decoding means for operating on the first output data stream to obtain N streams of PCM audio data respectively corresponding to the N-channel analog audio signals. PA1 N-channel analog-to-digital converter means for converting the N channels of PCM audio data to the N-channel analog audio signals, PA1 channel separator means for separating and processing the second output data stream into N streams of single bit sequence data respectively corresponding to the N-channel analog audio signals, and PA1 N low-pass filters for respectively converting the N streams of single bit sequence data to the N-channel analog audio signals.
As a specific example, using the single bit stream method with a sampling frequency which is 66 times the PCM sampling frequency of conventional audio CDs (44.1 kHz), the sampling frequency becomes (66.times.44.1 kHz)=2.82 MHz, i.e. the bit rate of the resultant data stream is 2.82 Mbps. In that case, the reproduction bandwidth is 100 kHz, and the signal/noise ratio of the reproduced analog audio signal is approximately 120 dB.
With the prior art PCM method on the other hand, to achieve such results, it would be necessary to use for example a sampling frequency of 200 kHz with 20 bits per sample, so that the transmission frequency of the resultant data would be 4.00 MHz. The superiority of the single bit stream method is therefore apparent.
It can thus be understood that adoption of the single bit stream method for recording audio signals on optical recording disks would enable the defects of prior art types of audio CD to be overcome, so that it can be expected that the single bit stream method will become widely adopted in the field of audio recording technology. However most of the digital audio recording and playback equipment which is now in existence is designed for use with the conventional PCM method, and of course most of the recorded media which are now available utilize the PCM data format. It would not be practicable, therefore, to expect the users of such optical recording disks to immediately switch over to use of the single bit stream method.
On the other hand, standards have already been established for DVDs, which are expected to become widely used in the field of multimedia, and DVD playback systems and AV (audio-video) software are already being marketed, based on the standards which have been established. It can thus be anticipated that DVDs will come into widespread use, as a high-density recording medium. Basic DVD standards which have been established include the DVD-Video standards, for video disk applications, the DVD-ROM standards, for ROM (read-only memory) and the DVD-RAM standards, for RAM (random access memory) applications. Respective standards for the data pack formats of these have been established.
Such data pack format standards are illustrated in FIGS. 20 to 23. FIG. 20 shows the basic format of a DVD data pack. FIG. 21 shows the format of a video pack which conveys video data, FIG. 22 shows the format of an audio pack, for the case in which audio signals are encoded as PCM data, FIG. 23(a) shows the format of a DSI (Data Search Information) video pack, and FIG. 23(b) shows the format of a VBI (Video Blanking Information) video pack.
It should be noted that the format standards for PCM data in audio packs are not limited to that shown for PCM data in FIG. 22. Other audio pack format standards include those established for audio which accompanies various types of video signal, e.g. for the Dolby AC-3 audio system (in accordance with the NTSC video system) system, and for the MPEG (Moving Pictures Experts Group) audio system (in accordance with the PAL, SECAM video systems).
Hence, considering the widespread popularity which can be anticipated for DVDs in the future, it would be very advantageous to be able to make the single bit stream method compatible with the existing DVD standards. In that way, the advantages of the single bit stream method with regard to increased reproduction bandwidth and improved signal/noise ratio could be made widely available to users. In particular, it should be possible to ensure compatibility with the PCM data format for audio signals, specified in the DVD standards.